Read all 6 of these pretty much on the trot, which is good and bad. The bad thing about it is that it becomes obvious that there are passages that are repeated in each book. For example, when ever he introduces Madeline to the book, he describes how she & Robert became acquainted - which you know if you're read the previous ones. But it's only a minor gripe - and certainly one you wouldn't notice to the same extent if you read them individually. Other than the the stories are quite interesting. It tends to be reasonably obvious who-dun-it, although that doesn't always detract from the story, the means and motivations being as interesting as the unravelling of the crime. Not the most taxing of books, but they're a reasonably easy read.